stiffen, as soon as the last lead has entered the former, by which the platinum will be more regularly distributed in the alloy, and cannot so well collect in different unconnected lumps, unexposed afterward to the action of the acid. The platinum may then be extracted and reduced from the solution, as above.

XIII.

Assay of Copper Ores-German and Hungarian Method.

THIS ore, if a sulphuret, as is very generally the case, should, after having been reduced to the finest possible powder, be submitted to the process of roasting, vulgarly termed calcining. For this purpose one part by weight of the ore is mixed up with one fifth of graphite, (black-lead,) which, consisting of carbon in a more condensed state than that element occurs in charcoal, is, therefore, so much the more effectual in driving off the sulphur. This mixture of the two should be exposed to an intense red-heat in the cupel, (fig. 1,) (painted over on the inside with red chalk, or Spanish red, to prevent adhesion,) for about twenty minutes, after which it is

to be taken out, and stirred up with a small iron ladle to expose the unburnt parts of graphite, when it should again be exposed in the muffle. In about a quarter of an hour we take it out again, pound it over, for the mass is generally clogged, and mix it with about twice its weight of charcoaldust, after which we continue the roasting for about one-half to a full hour, according as the ore contains a little or much sulphur, vapours of which may be seen rising during the whole of this process.

After this the ore has a reddish, or what is generally called a ferruginous colour, and we now take it from the first crucible, and introduce it into that represented in fig. 4, Plate II., or if not in possession of such, into a Hessian crucible, fig. 5, on the same plate. For this purpose, however, some black flux should be provided. It consists of carbonate of potash and lime, and is made by igniting together one part by weight of saltpetre, and two of com

mon tartar. The flux produced should be kept carefully corked to prevent the absorption of hygroscopic water. It is still better to make it only when required for immediate use.

If the ore is poor, one tenth part by weight of oxide of antimony, (antimonious acid,) or of arsenic, (arsenious acid,) or if it is richer, fifteen per-cent. of pure lead* are requisite, as will hereafter be seen to make the particles of copper unite. One of these together with three parts of black flux, one half the weight in borax, and two parts of table salt, must then be added to the roasted ore, though none except a part of the

*Some assayers use neither of these three, on the ground that the arsenic, as occasionally even 40 percent. may be taken, is very difficult to separate from the copper, and that the antimony may unite with part of the same, forming an antimoniate. They therefore only employ borax and black flux, in about the same proportions, however, as given above. It is hard to say which is best, and it must be left to the discretion and experience of the assayer to act as may be most suitable to his peculiar ores.

flux are mixed with the mineral now investigating. The salt is merely used to form a crust over the whole. When all have thus been placed in the crucible, they are covered over with a piece of charcoal, cut to match the size of the vessel; after which the cover is put on. We then expose it to a white-heat for about one-half to one full hour, as may be most convenient, either in the draught furnace, fig. 6, Plate I., or in the muffle furnace; in which latter case, we must allow the longest period of time.

The carbon of the flux is intended to reduce the peroxide of copper produced by roasting, while its carbonate of potash unites with the earthy contents of the ore and the oxides of other metals present, such as iron, which would otherwise also be reduced to their metallic state. With these it forms a slag, the borax being added to make it flow easy, and allow the copper to collect in one button.